1. Field of Invention
This invention generally relates to electronic measurement, specifically to calibration of radio frequency equipment such as can be used to perform measurements of radiated waves and other tasks.
2. Prior Art
Receiver Calibration
Previously, receivers were calibrated using signal generators that used switchable attenuators (functionally similar to concatenations of attenuators and having errors similar to concatenations of attenuators as described in BACKGROUND OF THE INVENTION—PRIOR ART, Weak Signal Generation in the other patent application Ser. No. 14/144,797 divided from U.S. Pat. No. 8,620,301, “Weak Signal Generation”.) Some signal generators have the ability to synthesize certain complex signals such as are used by modern communication receivers, though these signal generators are usually quite expensive. Such signal generators are often adequately shielded that the output signal can be attenuated to quite a weak strength without the output signal being overwhelmed by direct radiation from the signal generator to the receiver.
Sometimes the ability to synthesize a particular modulation in a signal generator is not practical at an acceptable price. In such a case it is more practical to use a transmitter designed to communicate using the desired protocols (including complex signals) as the signal source for receiver calibration. The signal is attenuated before being delivered to the receiver under test. Since a transmitter is designed to send signals at relatively high power levels, it is usually not shielded sufficiently for receiver test. Additional shielding must usually be supplied. Previously, that has often been done by housing one of the communicating units, usually the receiver, in a shielded room. Shielded rooms are expensive; hence, often not readily available.
Often, receivers that are designed for communication are designed for that and little more. They do not have the ability to measure signal properties, or can do so only with poor accuracy. This last fact leads to the site survey methods described in BACKGROUND OF THE INVENTION—PRIOR ART in U.S. Pat. No. 8,620,301.